Stress balanced semiconductor packages, method of fabrication and modified mold segment

ABSTRACT

Stress balanced semiconductor device packages, a method of forming, and a method of modifying a mold segment for use in the method. A semiconductor die is attached to one side of a substrate having discrete conductive elements such as a ball grid array (BGA) on the opposing side thereof. An envelope of encapsulant material is disposed over the semiconductor die on one side of the substrate while a stress balancing structure comprising at least one stem member and at least one transversely extending branch member formed of encapsulant material is disposed over the opposing side of the substrate in an arrangement which does not interfere with the discrete conductive elements. The envelope and the stress balancing structure may be simultaneously formed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/636,332,filed Aug. 6, 2003, pending, which is a divisional of application Ser.No. 10/227,329, filed Aug. 23, 2002, now U.S. Pat. No. 6,696,748, issuedFeb. 24, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the packaging of electronic componentssuch as integrated circuits or other electronic devices. In particular,this invention relates to an improved semiconductor device packagewherein at least one semiconductor die is encapsulated on a substrateand another volume of encapsulant material is added to the opposing sideof the substrate in a configuration to assist in control of the stressesin the package.

2. State of the Art

Conventionally, semiconductor dice have been packaged in plastic or,less commonly, in ceramic packages. Packages may support, protect, anddissipate heat from semiconductor dice. Packages may also provideexternal connective elements for providing power and signal distributionto and from semiconductor dice, as well as for facilitating electricaltesting, such as burn-in testing and circuit evaluation, ofsemiconductor dice prior to or after assembly thereof with higher-levelcomponents, such as carrier substrates or circuit boards.

FIG. 1A schematically illustrates a section of a conventionalboard-on-chip (BOC) semiconductor device assembly 100A with electricalcontacts which conventionally comprise a ball grid array (BGA) ofdiscrete conductive elements 12 such as solder balls. FIG. 1A showssubstrate 4, typically a printed circuit board, mounted to semiconductordie 2. Semiconductor die 2 is placed in electrical communication withsubstrate 4 by bond wires 6 extending between bond pads 3 ofsemiconductor die 2 and terminal pads 5 through slot 7 in substrate 4using conventional wire bonding techniques. Both semiconductor die 2 andbond wires 6 are encapsulated in a transfer molded, filled polymervolume of encapsulant material shown by die encapsulation region 8 andwire bond cap 10. FIG. 1B illustrates a side view and FIG. 1Cillustrates a plan view of the semiconductor device assembly 100A ofFIG. 1A.

FIG. 2A schematically illustrates a conventional chip-on-board (COB)assembly 200A fabricated using conventional wire bonding techniques.FIG. 2B shows a flip-chip configured semiconductor device assembly 200Bthat utilizes solder bumps 14 instead of wire bonds to electricallyconnect semiconductor die 2 to substrate 4.

A semiconductor die, the encapsulation material, the adhesives or otherbonding agents used to connect the semiconductor die to the substrate,the substrate, and the electrical connection mechanisms between thesemiconductor die and the substrate of a semiconductor device assemblyare usually each made from a different material or combination ofmaterials. These different materials usually have differentthermomechanical properties due to differing coefficients of thermalexpansion (CTE), which differences result in stresses developing duringmanufacture or use of the semiconductor device assembly, the latter dueto thermal cycling. Several problems can result during manufacture anduse of semiconductor device assemblies due to the development of thesethermomechanical-related stresses. For instance, bowing of thesemiconductor device assembly can occur due to internal bendingstresses, causing cracking in the encapsulant materials and subjectingcomponents to environmental degradation, debonding of the semiconductordie from the substrate, solder joint failure, or cracking in thesubstrate itself. Even if minimal bowing is manifested initially,residual tensile stresses can still be present, eventually resulting inthe same problems after a period of time due to thermal cycling-inducedfatigue experienced during normal operation.

Residual tensile stresses develop during manufacture from theaforementioned mismatch of material CTEs, as well as from shrinkage ofthe encapsulation material during curing and hardening thereof. FIG. 3shows an exaggerated view of bowing in a wire bonded BOC assembly 100Aas previously depicted in FIG. 1A due to the thermomechanical stresses.The bowing occurs in significant part due to the imbalance of the volumeof encapsulant material between the two opposing sides of substrate 4.As shown in FIG. 3, there is significantly more encapsulant materialvolume present in die encapsulation region 8 than in wire bond cap 10.Further, die encapsulation region 8 extends completely across substrate4, while wire bond cap 10 runs primarily longitudinally over substrate4, extending laterally only a sufficient distance to cover bond wires 6and slot 7. The resulting predominant tensile stress developed nearsubstrate 4 and on one side thereof due to shrinkage of thesubstantially different volumes and extents of the encapsulant materialapplied to the opposing sides of substrate 4 can cause significantproblems with respect to package integrity. One particularly notableproblem is cracking in wire bond cap 10, which exposes the wires toenvironmental degradation and may itself cause breakage of the thin,delicate bond wires 6.

The prior art has attempted to address the issues of undesirablestresses in semiconductor packaging. For example, U.S. Pat. No.5,627,407 to Suhir et al. purportedly solves the problem of unwantedthermomechanical stresses by using a thin “surrogate layer” on theentire substrate side opposite the encapsulated semiconductor die.However, this method utilizes significantly more material in thesurrogate layer than may be needed and involves the use of a differentmaterial than the encapsulant. Also, by covering an entire side of thesubstrate with the surrogate layer, it is difficult, if not impossible,to place discrete conductive elements, such as a BGA, on the substrateside bearing the surrogate layer. Further, adding this surrogate layermay be required to be performed as an additional process step. Littleguidance is provided as to how the encapsulant and the surrogate layermight be applied concurrently, as is indicated by Suhir as being adesirable approach. In addition, placement of a surrogate layerproximate each substrate and prior to encapsulation will itself requireadditional cost and alignment considerations.

U.S. Pat. No. 6,294,831 to Shishido et al. attempts to reduce bowing ina flip-chip type semiconductor device assembly by bonding a structureover the back side of a flip-chip configured semiconductor die and onthe opposing side of the semiconductor die to an interposer substrate towhich the semiconductor die is mechanically secured and electricallyconnected, the structure having a CTE similar to that of the substrate.

U.S. Pat. No. 6,291,899 to Wensel et al. addresses bowing in a COB BGAsemiconductor device assembly by applying a so-called stabilizing plateto a side of the substrate opposite that to which the semiconductor dieis back side-attached and wire bonded. The stabilizing plate is formedof a rigid material different from that used to encapsulate thesemiconductor die and is applied before the encapsulant is applied overthe semiconductor die. Placement of the stabilizing plate on the side ofthe substrate carrying the discrete conductive elements of the BGA alsorequires fairly precise alignment of the stabilizing plate duringplacement.

Other U.S. Patents have addressed the issue of stresses in semiconductorpackages but only with regard to bowing in lead frame assemblies. Forexample, U.S. Pat. No. 6,384,487 to Smith and U.S. Pat. No. 6,258,624 toCorisis attempt to equalize the volume of encapsulant material on bothsides of a lead frame to minimize bowing. U.S. Pat. No. 6,316,829 toBoon et al. attempts to solve the same problem, but by molding groovesand ridges in an encapsulated lead frame.

While the prior art has attempted to address bowing and otherstress-related problems in semiconductor device assemblies, a needexists for a semiconductor device assembly of a design whereby thestresses in the assembly can be controlled effectively while notrequiring added process steps or the use of substantial additionalmaterials, expensive materials or complex structural configurations,each of which increase fabrication cost. Further, it would be desirableto provide a semiconductor device assembly design which easilyaccommodates the use of a BGA for connection to higher-level packaging.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a structural configuration whichsubstantially balances stresses in semiconductor device assembliesthrough selective placement of encapsulant material on opposing sides ofa substrate. The semiconductor device assembly includes at least onesemiconductor die mounted to a substrate in either a board-on-chip (BOC)or chip-on-board (COB) configuration. Flip-chip, wire bonding, or anyother technique known in the art may be used to electrically connect thesemiconductor die to the substrate. The semiconductor die is attachedand electrically connected to one side of a substrate with discreteconductive elements such as a ball grid array (BGA) on the opposing sideof the substrate.

In a broad embodiment, at least one stem member and at least onetransversely extending branch member may be placed on the side of acarrier substrate such as an interposer substrate opposite the side onwhich a semiconductor die and die encapsulation region extending overthem are placed. The combined volumes of the stem and branch member maybe substantially equal to the volume of the die encapsulation region.

One exemplary embodiment of the present invention comprises a wirebonded BOC semiconductor device assembly where the wire bonds extendingbetween the bond pads of the semiconductor die and terminal pads of thesubstrate are encapsulated with a longitudinally extending wire bond capin a conventional manner with at least one laterally extending branchmember of encapsulant material formed over the same side of thesubstrate as the wire bond cap. The wire bond cap and at least onebranch member may be formed simultaneously with an encapsulation regionformed over the semiconductor die.

Another exemplary embodiment of the present invention comprises a wirebonded COB assembly wherein the semiconductor die and wire bonds areencapsulated on one side of the substrate and at least one additionallongitudinally extending stem member having at least one laterallyextending branch member of encapsulant material may be formed on theopposing side of the substrate and simultaneously with encapsulation ofthe semiconductor die and wire bonds.

Yet another exemplary embodiment of the present invention comprises asemiconductor device package wherein a flip-chip configuredsemiconductor die is attached and electrically connected to a substrate.The semiconductor die may be encapsulated on one side of the substratesimultaneously with application of encapsulant material to the opposingside of the substrate in a configuration providing at least onelongitudinal member and at least one laterally extending member.

The present invention offers the advantage, among others, of reducinginternal bending stresses on an encapsulated semiconductor deviceassembly by moving the neutral axis, where such bending stresses areslight to nonexistent, closer to or even within the center of thesubstrate. Further, the stress-balancing structures of the presentinvention are formed using an insubstantial volume of additionalencapsulant material and simultaneously with molding of an encapsulantenvelope over the semiconductor die and, if applicable, a wire bond cap.

The present invention also encompasses a method of making the abovesemiconductor packages and a method of modifying a mold segment for usein the method.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, which illustrate what is currently considered to be thebest mode for carrying out the invention and in which like elements andfeatures are identified by like reference numerals:

FIG. 1A is a schematic sectional view of a conventional BOCsemiconductor device assembly utilizing wire bonding;

FIG. 1B is a side elevation of a conventional BOC semiconductor deviceassembly utilizing wire bonding;

FIG. 1C is a plan view of the conventional BOC semiconductor deviceassembly of FIG. 1B;

FIG. 2A is a schematic sectional view of a conventional COBsemiconductor device assembly utilizing wire bonding;

FIG. 2B is a schematic sectional view of a conventional COBsemiconductor device utilizing a flip-chip configured die;

FIG. 3 is a schematic sectional view of the semiconductor deviceassembly of FIG. 1A bowed under stress;

FIG. 4A is a side elevation of a stress balanced semiconductor deviceassembly according to the present invention;

FIG. 4B is a plan view of the stress balanced semiconductor deviceassembly of FIG. 4A;

FIG. 4C is a side sectional view of a wire bonded BOC stress balancedsemiconductor device assembly having an external configuration inaccordance with FIGS. 4A and 4B;

FIG. 4D is a side sectional view of a wire bonded COB stress balancedsemiconductor device assembly having an external configuration inaccordance with FIGS. 4A and 4B;

FIG. 4E is a side sectional view of a flip-chip semiconductor deviceassembly on an interposer substrate having an external configuration inaccordance with FIGS. 4A and 4B;

FIG. 4F is a plan view of another external configuration of a stressbalanced semiconductor device assembly suitable for use with the COBsemiconductor device assemblies of FIGS. 4D and 4E;

FIG. 5 is a schematic elevation of a memory module including a pluralityof stress balanced semiconductor device assemblies according to thepresent invention; and

FIG. 6 is a computer system including at least one stress balancedsemiconductor device assembly according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 4A-E, at least one semiconductor die 2 is mounted byconventional methods such as adhesive or other bonding agent (and, insome instances, by a flip-chip connection) to form either achip-on-board (COB) or board-on-chip (BOC) assembly with substrate 4.Substrate 4 may be a printed circuit board formed, for example, of BTresin or comprising an FR-4 or FR-5 laminate, a ceramic substrate, asilicon substrate, a flexible circuit board, or any other type of rigidor flexible circuit board material known to one of ordinary skill in theart. Semiconductor die 2 is encapsulated with a material as shown by dieencapsulation region 8 on a first side of substrate 4. Discreteconductive elements 12 are provided on the side of substrate 4 oppositesemiconductor die 2. Discrete conductive elements 12 may be arranged ina ball grid array or other suitable configuration for mechanically andelectrically connecting the semiconductor device assembly tohigher-level packaging.

In one embodiment, as shown in FIGS. 4A and 4B, an assembly 400 is usedwhere a stem member 9 and at least one branch member 11 extending fromstem member 9 are provided adjacent to discrete conductive elements 12.Stem member 9 extends in a longitudinal direction and branch member 11extends in a transverse direction. FIG. 4B shows a preferred geometrywhere stem member 9 extends longitudinally between two portions of anarray of discrete conductive elements 12 and two branch members 11extend in a transverse direction at each end of the array of discreteconductive elements 12. Further and as shown, stem member 9 and branchmember 11 may be substantially perpendicular to each other. In oneembodiment, stem member 9 and branch member 11 are formed at the sametime and form an integral structure.

In an exemplary embodiment as shown in FIG. 4C employing the externalconfiguration shown in FIGS. 4A and 4B, a BOC assembly 400C is usedwhere semiconductor die 2 is electrically connected to substrate 4 usingbond wires 6. Bond wires 6 are extended between bond pads 3 ofsemiconductor die 2 and terminal pads 5 of substrate 4 through slot 7using conventional wire bonding techniques known to one skilled in theart. As in the other exemplary embodiments, substrate 4 may be a printedcircuit board, flexible circuit board, or any other type of circuitboard known to one skilled in the art. Discrete conductive elements 12are provided on the side of substrate 4 opposite semiconductor die 2.Discrete conductive elements 12 may comprise an array such as a BGA.Semiconductor die 2 is encapsulated by die encapsulation region 8. Bondwires 6 are encapsulated by a wire bond cap 10 which, according to thepresent invention, also comprises a longitudinally extending stem member9 along substrate 4 adjacent to and between two portions of the array ofdiscrete conductive elements 12. At least one branch member 11 mayextend in a direction substantially transverse to stem member 9 andlaterally across substrate 4. Two branch members 11 may respectively liepreferably on opposing sides of discrete conductive elements 12. In thisembodiment, combined wire bond cap 10 and stem member 9 and contiguousbranch members 11 are preferably formed at the same time andsimultaneously with formation of die encapsulation region 8 to form anintegral assembly. It is also currently preferred that wire bond cap10/stem member 9, branch members 11 and die encapsulation region 8 beformed in a transfer molding process wherein a molten, silicon-filledthermoplastic polymer dielectric mold compound is molded about the BOCassembly in a transfer mold.

Such a transfer mold may comprise two mold segments, one configured witha mold cavity to form die encapsulation region 8 and the opposing oneconfigured to define wire bond cap 10/stem member 9 and branch members11. Both mold segments will, of course, sealingly engage a surface ofsubstrate 4 to preclude mold compound bleeding onto unwanted areas ofsubstrate 4. It should be noted that the embodiments shown in FIGS.4A-4F have the advantage of not having to introduce additional processsteps since bond wires 6 are typically encapsulated in conventional BOCwire bonded assemblies (i.e., a wire bond cap) simultaneously with dieencapsulation region 8 and, in the other embodiments, an appropriatemold segment may be configured and employed in the molding processalready in use.

The modification of an existing mold segment defining the wire bond cap10/longitudinal stem member 9 in the embodiment of FIG. 4C to includemold cavity portions to define laterally extending branch members 11configured as shown in FIG. 4B may be easily effected using, forexample, a milling machine or electrodischarge machining (EDM), amongother techniques. Of course, additional runners and vents may be addedto the mold segment as desired or required. Accordingly, once the moldsegment is so modified, the only additional cost in the BOC assemblyencapsulation process is the minimal cost of the additional moldcompound used to define the branch members 11. Of course, a transfermolding mold is normally configured for molding of a large number ofsemiconductor device assemblies and the mold segments used to definewire bond cap 10/stem member 9 for each of such assemblies may bemodified to the configuration of the present invention.

FIG. 4D depicts a COB assembly 400D in accordance with the presentinvention, wherein semiconductor die 2 is back-bonded to carriersubstrate 4, and die encapsulation region 8 extends over bond wires 6facing away from substrate 4. Discrete conductive elements 12 areoperably coupled to bond wires 6 through vias and, if desired orrequired, a redistribution layer (RDL) extending over a surface ofsubstrate 4 or redistribution traces extending therewithin. Longitudinalstem member 9 and branch members 11 are arranged as depicted in FIG. 4B,although this arrangement is not required. For example, stem member 9may be eliminated and one or more branch members 11 used alone.Alternatively, as shown in FIG. 4F, two longitudinal stem members 9 maybe placed along parallel, opposing edges of substrate 4, with branchmembers 11 placed along parallel, opposing edges of substrate 4 orientedperpendicular to the other two edges, stem members 9 and branch members11 framing an array wherein a BGA of discrete conductive elements 12 maybe placed. Thus, a mold segment configured to achieve the foregoing stemmember and branch member configuration would be placed against the sideof substrate 4 opposite to semiconductor die 2.

FIG. 4E shows a COB assembly 400E using a flip-chip type semiconductordie 2 having solder bumps or other discrete conductive elements 14extending from an active surface of semiconductor die 2 to mechanicallyand electrically connect semiconductor die 2 to substrate 4. As with theother embodiments, the BGA side of substrate 4 may be configured asdepicted in FIG. 4B or 4F.

In all embodiments shown in FIGS. 4A-4F, the resulting neutral axis ofthe semiconductor package is preferably located to be at least near, ifnot coincident with, the center of substrate 4. That is to say, whilethe substrate extends primarily in the X-Y plane, it also has a depth orthickness transverse to the X-Y plane which is commonly referenced asthe “Z” dimension of the substrate. Thus, the present inventiondesirably places the neutral axis, or location wherein internal bendingstresses are negligible to nonexistent, within the center of thesubstrate. This design has the advantage of substantially lowering oreven eliminating tensile stresses near the substrate, reducing thetendency of the semiconductor package to bow, reducing the tendency ofcracking in stem member 9, and eliminating or reducing the tendency ofthe assembly to exhibit the aforementioned structural problems. Stemmember or members 9 and branch member or members 11 may also provideadditional structural support for the semiconductor package.

Further, in all embodiments shown by FIGS. 4A-F, die encapsulationregion 8, stem member 9, and branch member or members 11 may be formedfrom materials known in the art and the encapsulant material used may bethe same for both sides of substrate 4. For example, polymers such asepoxies, silicones, silicone-carbon resins (SYNCAR™), polyimides, orpolyurethanes may be used. Composite materials such as reinforcedpolymers may also be selected for die encapsulation region 8, stemmember 9, and branch member 11. Die encapsulation region 8, stem member9, and branch member or members 11 may be formed from molding techniquesknown in the art other than transfer molding, including, for example,pot molding and injection molding. Other methods and materials toproduce die encapsulation region 8, stem member 9, and branch member 11will be readily apparent to those of ordinary skill in the art.

It will also be appreciated that the present invention provides, throughthe presence of at least one stem member and at least one branch member,a stabilizing structure for the BGA and a standoff for discreteconductive elements 12 thereof.

Referring now to FIG. 5, a memory device or module 500 is shown whichincorporates a plurality of semiconductor dice 2 packaged according tothe present invention. The memory device 500 includes a printed circuitboard 16 to which the one or more packaged semiconductor dice 2 may bemechanically and electrically operably coupled therewith to form amemory module. A plurality of electrical edge connectors 18 may beformed on the printed circuit board 16 to provide input and outputconnections from an external device, such as, for example, a motherboardof a computer, to the one or more semiconductor dice 2.

Referring now to FIG. 6, a computer system 600 is shown which includes aprinted circuit board 16 such as, for example, a motherboard. Theprinted circuit board 16 may be operably coupled to at least oneprocessor 20, such as, for example, a central processing unit, (CPU) andat least one memory device 500. The memory device 500 may include one ormore semiconductor dice 2 packaged as described above in the presentinvention. The printed circuit board 16 is operably coupled with atleast one input device 22 such as, for example, a keyboard, a mouse, asensor or another computing device. The printed circuit board 16 is alsooperably coupled with at least one output device 24 such as, forexample, a printer, a monitor, an actuator or another computing device.

Although the foregoing description contains many specifics, these arenot to be construed as limiting the scope of the present invention, butmerely as providing certain exemplary embodiments. Similarly, otherembodiments of the invention may be devised which do not depart from thespirit or scope of the present invention. The scope of the invention is,therefore, indicated and limited only by the appended claims and theirlegal equivalents, rather than by the foregoing description. Alladditions, deletions, and modifications to the invention, as disclosedherein, which fall within the meaning and scope of the claims areencompassed by the present invention.

1. A computer system comprising: a printed circuit board; a processoroperably coupled to the printed circuit board; at least one input deviceoperably coupled to the printed circuit board; at least one outputdevice operably coupled to the printed circuit board; and at least onememory device operably coupled to the printed circuit board, the atleast one memory device comprising: a substrate; at least onesemiconductor die disposed on a first side of the substrate; a pluralityof discrete conductive elements disposed on an opposing side of thesubstrate and operably coupled to the at least one semiconductor die; anencapsulation region extending over the at least one semiconductor die;and at least one stem member and at least one branch member extendingsubstantially transversely to the at least one stem member, the at leastone stem member and the at least one branch member disposed on theopposing side of the substrate.
 2. The computer system of claim 1,wherein the at least one memory device comprises a plurality of memorydevices.
 3. The computer system of claim 2, wherein the plurality ofmemory devices is configured as a memory module.
 4. The computer systemof claim 1, wherein the at least one stem member and the at least onebranch member are substantially perpendicular to each other.
 5. Thecomputer system of claim. 1, wherein the at least one stem member andthe at least one branch member are substantially contiguous.
 6. Thecomputer system of claim 1, wherein the at least one stem member isdisposed between portions of the plurality of discrete conductiveelements and the at least one branch member is disposed at an end of theplurality of discrete conductive elements.
 7. The computer system ofclaim 6, wherein the at least one branch member comprises two branchmembers, each disposed at an end of the plurality of discrete conductiveelements.
 8. The computer system of claim 1, wherein the at least onestem member is disposed at one side of the plurality of discreteconductive elements and the at least one branch member is disposed at anend of the plurality of discrete conductive elements.
 9. The computersystem of claim 1, wherein the at least one stem member comprises twostem members, each disposed at one side of the plurality of discreteconductive elements and the at least one branch member comprises twobranch members, each disposed at one end of the plurality of discreteconductive elements.
 10. The computer system of claim 1, wherein the atleast one stem member and the at least one branch member are made from apolymer material.
 11. The computer system of claim 10, wherein thepolymer material comprises a silicon particle-filled thermoplasticpolymer.
 12. The computer system of claim 10, wherein the polymermaterial is selected from the group consisting of epoxies, silicones,silicone-carbon resins, polyimides, and polyurethanes.
 13. The computersystem of claim 10, wherein the die encapsulation region is formed ofthe same polymer material as the at least one stem member and the atleast one branch member.
 14. The computer system of claim 1, wherein theat least one semiconductor die includes an active surface facing thesubstrate.
 15. The computer system of claim 1, wherein the at least onesemiconductor die includes an active surface facing away from thesubstrate.
 16. The computer system of claim 1, wherein the at least onesemiconductor die is operably coupled to the plurality of discreteconductive elements at least partially through bond wires.
 17. Thecomputer system of claim 16, wherein the bond wires extend through aslot in the substrate.
 18. The computer system of claim 1, wherein thepackage includes a neutral axis at least proximate to a center of thesubstrate.
 19. The computer system of claim 18, wherein the neutral axislies substantially in a plane of the substrate.
 20. The computer systemof claim 1, wherein the at least one stem member is substantiallylongitudinally oriented on the substrate and the at least one branchmember is substantially laterally oriented on the substrate.
 21. Thecomputer system of claim 1, wherein a volume of material of the dieencapsulation region and combined volumes of the at least one stemmember and the at least one branch member are substantially equal. 22.The computer system of claim 1, wherein the at least one stem membercomprises a wire bond cap.